1. Field of the Invention
Electro-medical electrodes for establishing electrical connection between an animal body, specifically the human body, and an electrical recording device.
2. Description of the Prior Art
In the prior medical art it has been a well known practice to attach electrodes to the skin to provide contact and through which electrical signals are transmitted between the anatomy and electromedical apparatus. U.S. Pat. to Baum, No. 3,187,745 and Berman, No. 3,085,577 are exemplary of such electrodes, employed in the art of electrocardiography wherein electrical heart signals are received, and the U.S. Pat. to Alderman, No. 2,872,926, is exemplary of an electrode employed in the art of electroencephalography wherein electrical brain signals are received. The signals, in either event, are suitably recorded or charted and are a measure of a body function or functions under known conditions, such as rest, exercise, mental stress, intensive care and the like. Also, with the advent of space travel and radio monitoring, or with the use of small portable electromedical recording devices, it is now possible to biologically monitor body functions of pilots under conditions encountered in fulfilling their missions. The importance of maintaining a low resistance contact with the anatomy is well recognized and the patents referred to are exemplary of obtaining the desired results by utilizing electrolytes, electroconductive gels, pastes and/or adhesives through which an electric signal is transmitted. Other pertinent U.S. Pat. Nos. are 2,555,037; 2,985,172; 3,170,459; 3,474,775; 3,475,213; 3,565,059; 3,607,788; 3,710,782; 3,713,435; and 3,911,906. The U.S. Pat. to Howell, No. 2,943,628 is exemplary of another approach wherein contact is made with metal foil surrounded by a pressure sensitive adhesive.
As the art referred to has advanced, certain improvements appear desirable, including the need for more rapid and expedient application of the electrodes and simplification of their construction, together with its attendant reduction of cost. Additionally, a light weight electrode is desirable to minimize or obviate inertial movement relative to a predetermined position on the skin.
Prior art biological electrodes for electrocardiography are not capable of rapid attachment to the body and rapid removal therefrom, are not non-irritating, transparent to medical x-rays, or easily removed by dissolution of adhesive or by peeling. The adhesive used with prior art biological electrodes, such as gelled pads, has a tendency to dry out during storage or use, and in some cases serves as a nutrient for bacteria.
While others have taught the incorporation of conducting particles into electrode surfaces, the absence from their teaching of several critical elements prevented them from disclosing products of practical utility. A satisfactory biological electrode must exhibit at least two required properties; (1) it must provide a high electrical conductivity through the skin, and (2) it must have enough adhesive strength to remain securely in place on the patient. It has been found that the use of particles of conventional geometry, i.e., "fine powder carbon" (Reinhold U.S. Pat. No. 3,911,906), silver flakes or their metal particles (Burton U.S. Pat. No. 4,008,721), and carbon black (Johnson U.S. Pat. No. 3,474,775), results in pressure sensitive electrodes which may have one, but never both of the above properties. Prior electrodes having satisfactory conductivity did not have enough adhesive strength, and increasing the adhesive content to cure this defect resulted in lowering the conductivity to an unsatisfactory level.
Stow U.S. Pat. No. (3,475,213) overcome part of the problem with the use of particles having a "substantial thickness in relation to their width and length", typically "spheres or granules". While solving part of the problem, particles of this geometry still leave conductivity across the face of the electrode so low as to require a conductive backing material. This backing material would typically be a metal foil, and would not lend itself well to the skin "breathability" or conformability desirable in a biological electrode.
The present invention, which is described hereinafter, takes advantage of the unique geometry of fibers used with a "soft" (deformable on a microscopic scale) adhesive to solve the problems not addressed by the prior art. In contrast to the particles used by Stow, fibers have a high length-to-thickness (diameter) ratio, and thus a single fiber may make physical and electrical contact with the many other fibers that may cross it anywhere along its length. This property of fibers provides a very high conductivity at low fiber concentrations in the adhesive, thus permitting the construction of an electrode having good adhesive strength, good electrical properties, and no foil backing. The particles mentioned in the prior art, on the other hand, can make contact with only a few other particles, and then only in the confines of a non-linear geometry, resulting in the limitations already discussed.
For use as a biological electrode, conductivity through the electrode is important, but is not enough in itself; the electrode surface must provide for high conductivity to the skin and into the body. Fibers imbedded in a rigid or semirigid binder might have excellent electrical properties throughout the electrode, but would not be able to provide the many necessary skin contacts to make a usable biological electrode interface. To facilitate good contact between the skin and fibers, the adhesive or binder in the electrode must be soft enough at the time of application to allow the adhesive to "flow" out of the way of the fibers, permitting as many fibers as possible to make direct contact with the skin. The adhesive then fills in any available voids provided by skin and fiber irregularities to provide for firm holding strength. The result is a series of microscopic hills (fibers) for conductivity, with adhesive in the valleys for holding strength. Thus the combination of fibers and the required type adhesive is important.
The above desired properties for the electrode of the present invention are obtained in one of two ways: (1) combining fibers with a soft, pressure sensitive adhesive, or (2) combining fibers with a solvent activated adhesive that is soft upon application to the skin, but becomes more rigid upon drying. For optimum properties, the fibers should have a very small diameter (.apprxeq.0.0003") and as high a length to diameter ratio as possible (preferably greater than 1500) for the following reasons. The small diameter of a fiber allows for more fibers per unit area of electrode surface, and thus better electrical properties. The greater the length of the fibers involved, the fewer fiber ends will be present on the electrode surface, resulting in less skin irritation to the patient. Unacceptable skin irritations and itching has been observed when 0.0003" diameter fibers are used in lengths much shorter than one-half inch, but very little irritation when longer fibers are used. Accordingly, fibers less than 0.0003" in diameter and at least one-half inch in length are preferred.